1. Field of the Invention
The present invention generally relates to optical transmission apparatuses, and particularly relates to a small blade-type optical transmission apparatus.
2. Description of the Related Art
As its name “blade” suggests, a blade-type optical transmission apparatus has the functions of a large transmission apparatus contained in a single blade where these functions are provided by a plurality of types of conventional units. As an example of such a blade, a case called an “enclosure” having 3U size (U indicates a rack size) contains 6 to 20 planes so as to provide a high-density and large-capacity optical communication apparatus.
A rapid dissemination of network technology that we saw in a past few years is still expanding. In a metropolitan area, especially, there is an increasing demand for the high-density implementation of a main-trunk optical transmission apparatus that provides a basis for such network technology. Against this background, lateral-stacking-type apparatuses of 1U size as well as box-shape apparatuses called pizza boxes were developed. In order to answer the call for super-higher density implementation, further, the blade-type optical transmission apparatus has emerged as a new form of a transmission apparatus.
In metropolitan areas, the cost of a place where transmission apparatuses are installed is a problem. In general, a rent for a unit area is higher for the office space of carriers (telephone companies) than for ordinary office space because the carriers tend to use places and business offices that are provided with tighter security and better disaster control. If a large area size is necessary for the installment of a transmission apparatus, the cost associated with the land increases as the number of apparatuses increases. This ultimately results in an increase in the price of provided services. In consideration of this, carriers seek to maintain competitive cost advantages by reducing an area size for installment while keeping necessary communications-system performance. In response to these needs, the lateral-stacking-type apparatuses of rack mounting and the box-shape apparatuses called “pizza box” are developed.
With the advent of these apparatuses, the maximum number of units mountable per rack has doubled, resulting in a significant drop of the cost associated with a place necessary for the installment of a transmission system. The miniaturization and thinning of these apparatuses are achieved by limiting channel capacity, functionality, expandability, etc. As a recent trend, a “pay-as-you-grow” method has been attracting attention, in which the number of channels is increased in response to a demand increase while installing a WDM transmission system with a small number of channels at the beginning in order to reduce an initial investment.
These apparatuses may be stacked one over another, or may be arranged side by side as many as they are necessary. When priority is given to the higher density of implementation per rack, however, there is space that can still be saved. For example, these lateral-stacking-type or box type apparatuses are provided with an LED, a craft port, a LAN port, an alarm output, etc., for the maintenance purpose. They are necessary, however, only when setting up or maintenance is carried out. Some circuit components may not be necessary for each of the apparatuses. The blade-type optical transmission apparatus was developed with an aim of achieving higher density by eliminating such waste as described above.
FIG. 1 is an illustrative drawing showing the construction of a related-art optical transmission apparatus of a divided-shelf type. In FIG. 1, an HS (high speed) shelf accommodates high-speed network signals. An XC (cross-connect) shelf cross-connects signals. A LS (low speed) shelf accommodates low-speed service signals. A TC (timing complex) shelf attends to clock control with respect to a network and inside the apparatus. An MC (management complex) shelf performs the monitor control of the network and the apparatus. In this manner, modules (shelves) are provided on a function-by-function basis, and are put together to form an apparatus that satisfies the needs of the system.
FIG. 2 is an illustrative drawing showing the construction of a single-shelf-type optical transmission apparatus of a related-art metro system. FIG. 3 is a block diagram of the optical transmission apparatus of FIG. 3. In the figures, LS units and XC units are coupled through a back plane (BP), and the XC units and HS units are coupled one-to-one through the back plane BP. In FIG. 3, an upward direction, i.e., from the LS units to the HS unit through the XC unit, is indicated by arrows. A downward direction, i.e., from the HS unit, the XC unit, to the LS units, is connected through signal lines provided on the back plane.
Further, Patent Document 1, for example, discloses an apparatus in which boards carrying electrical circuits thereon are provided in a bookshelf form, and signals output from each board are connected through a back plane. Patent Document 2 discloses replacing electrical connections between a plurality of panels through a backboard with optical connections through optical paths, wherein the optical paths are arranged in a loop form.                [Patent Document 1]        Japanese Patent No. 3348757        [Patent Document 2]        Japanese Patent No. 2606612        
A related-art optical transmission apparatus of the divided-shelf type tends to have an extremely large size, and requires a large initial investment because all the shelves are necessary even for an initial-stage configuration having a small number of channels. Further, it is not possible to adopt the “pay-as-you-grow” method in which the number of accommodated channels and the capacity of cross-connects are increased in response to a demand increase in the future. Moreover, the construction of the apparatus is extremely complicated, as exemplified by the provision of an interface circuit for connecting between the shelves that are divided.
In the single-shelf-type optical transmission apparatus of the related-art metro system, the back plane needs a number of signal pins sufficient to cope with the maximum capacity of the XC unit (i.e., the maximum number of the HS units and the LS units). Furthermore, even when the number of LS units is minimum, an XC unit is always necessary. Since the units of this apparatus are divided also on a function-by-function basis, a common circuit such as an XC unit is required in the initial configuration even if the configuration includes a small number of channels. This results in a high initial investment.
In this manner, the related-art divided-shelf-type or single-shelf-type configuration has a drawback in that the bulky size of apparatus makes it difficult to install the apparatus in limited space. Moreover, all types of units are necessary even for a minimum-number-of-channel configuration used at an initial stage after installment, and there is also a need to set aside spare units for expanding channels in the future. This gives rise to a problem in that the cost associated with the initial installment is high (i.e., profitability is low).
Moreover, since the units provided in the shelves are divided on a function-by-function basis rather than on a channel-capacity basis, the “pay-as-you-grow” method in which the number of accommodated channels and the capacity of cross-connects are increased in response to a subsequent demand increase is not applicable. Since the XC unit is required from an early stage after installment regardless of the number of accommodated channels, the cost of the apparatus is high despite the construction having a small number of channels. Furthermore, the construction of the apparatus is complicated, as exemplified by the provision of an interface circuit for connecting between the shelves that are divided. This results in a cost increase and an increase in the occurrence of failures.
In order to reduce the size of an apparatus, it is necessary to contrive ways to reduce the number of signal lines on the back plane. If all units are connected point-to-point in a matrix form, the number of signal lines becomes enormous, resulting in an enormous number of connector pins. Moreover, the use of a bus connection between units gives rise to a problem in that any given unit cannot transmit signals while another unit is occupying the bus connecting between the units.
Accordingly, there is a need for a blade-type optical transmission apparatus which facilitates the efficient use of space for installment and reduces the cost associated with the early stages of installment.